Armed pirates stormed a U.S.-flagged ship off the coast of Nigeria, separated the crew by nationality and kidnapped two Americans on board, a Pentagon official told ABC News.

The attack occurred during the early daylight hours today just inside international waters. The unknown number of assailants damaged the communications and navigation equipment on the ship as they departed with their captives, the American captain and chief engineer, possibly an effort to delay notification to authorities, the official said.

The men are likely now on land and the FBI in New York, which deals with Africa cases, is taking point in the investigation. Another U.S. government official briefed on the situation said American authorities are treating the case as a potential kidnapping-for-ransom and not a politically-motivated act.

Representatives for the State Department and the White House said they are "closely monitoring" the situation. The State Department said that as of now, there's no indication the attack was an act of terrorism.

The Americans were taken off the U.S.-flagged C-Retriever, a 222-foot vessel owned by U.S. marine transport group Edison Chouest Offshore, an oil supply vessel. Edison Chouest Offshore did not immediately return requests for comment.

"If you take the Americans, you get a good price, but at the same time you bring a lot of heat on you too," said Jack Cloonan, a former senior FBI agent and now works with Clayton Consultants, which specializes in international piracy, kidnapping and extortion . "The initial demands will probably be ridiculously high and you can infer from that who you are dealing with. Are these people skilled? Do they have professional negotiators?... Do we know who this group is and is their end game actually money? Because if it is, I'm happy. I'm pleased and now I know it can be a negotiable end."

The U.S. government has a longstanding policy against paying ransom for kidnapped citizens, but Cloonan said American law enforcement and intelligence agencies likely will be "deeply involved on multiple levels" during negotiations.

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The kidnapping was first reported by gCaptain.com, a maritime industry news website.

While globally piracy was down last year to its lowest levels in seven years, cases in Nigeria are on the rise. According to the International Maritime Bureau, pirate attacks off Nigeria's coast have jumped by a third this year -- allegedly perpetrated by criminal gangs who are looking for cargo ships with commodities, and seeking ransom for hostages. Around the world there have already been more than 200 "incidents" involving piracy this year, including 11 hijackings, the IMB said.

In 2009, on the other side of Africa, American Capt. Richard Phillips was held hostage on the Indian Ocean by a group of Somali pirates for five days before he was freed in a daring rescue by U.S. Navy SEALs. Hollywood recreated that ordeal in a recent blockbuster film starring Tom Hanks as Capt. Phillips.